1.1 Field of the Invention
The present invention relates generally to a method and an apparatus for the reliable detection and quantification of the flow rate produced by a leak from pressurized pipeline systems containing petroleum, solvent, or other chemical liquids.
1.2 Brief Discussion of the Prior Art
There are a wide variety of pressurized pipeline systems carrying petroleum, solvents, and other chemical products that may contaminate or seriously damage the surrounding environment in the event of a leak. In underground or underwater pipelines, where visual inspection is not possible, a leak can be a significant problem. Small leaks in these pipeline systems (e.g., several tenths of a gallon per hour) can go undetected for long periods of time and results in a large cumulative release of product into the soil or groundwater, or into fresh or ocean water.
The need for leak detection capability in pressurized pipelines associated with underground storage tanks containing petroleum products has recently been identified. This need is an important one because the number of tanks involved is very large, and so is the volume of product dispensed through the pipelines associated with these tanks. The pipeline systems in question are most commonly made of steel or fiberglass; they are typically 2 in. in diameter, 50 to 200 ft long, buried 1.5 to 3 ft below grade, and are pressurized at 20 to 40 psi while product is being dispensed. In September 1989, the United States Environmental Protection Agency (EPA) issued technical standards for the detection of leaks in underground storage tanks containing petroleum or other hazardous chemicals and solvents. This regulation established the minimum performance standards that must be met by any leak detection system designed for testing the integrity of underground tanks and/or the pressurized pipelines associated with these tanks.
The EPA requires that underground storage tank (UST) pipeline systems that contain petroleum products be tested for leaks either on a monthly or an annual basis. To satisfy the criterion for monthly testing, a system must have the capability to detect leaks as small as 0.20 gal/h with a probability of detection (P.sub.D) of 0.95 and a probability of false alarm (P.sub.FA) of 0.05. To satisfy the criterion for annual testing, a system must be capable of detecting leaks as small as 0.10 gal/h with the same P.sub.D and P.sub.FA required of the monthly test.
There have been a number of approaches to leak detection in pipeline systems. Some leak detection systems are designed to operate while product is being moved through the line; others require that the flow of product be stopped for the duration of a test. Leak detection systems generally use one of three methods: they measure the drop in pressure in the pipeline over a period of time, they measure the difference in pressure or flow rate at two or more points along the pipeline, or they measure the change in the volume of the product over a period of time. Detecting small leaks is difficult because there are many physical phenomena present in pressurized pipeline systems that produce pressure, volume, and flow-rate fluctuations that are as large as or larger than those produced by a leak. These normally occurring fluctuations degrade the performance of the leak detection system and result in false alarms or missed detections. As a consequence, a number of compensation schemes have been proposed to reduce them.